When selecting a CPU architecture for a web-server, you do need to know what you're doing with the server itself. Only you can tell if what you're doing can take advantage of multiple cores, or if it is solidly single-threaded. PassMark's PerformanceTest product test distils CPU performance down to a single number, yet what you're doing with your web-server is (likely) going to be a small subset of the CPU's overall capabilities.
Webservers tend to involve quite a bit of integer maths in what they do, though your application code will make all the difference. Secondly, webserver processes usually parallelize well (though not always, app-code makes a big difference here too). Because of this, they can exercise a CPU in ways that desktop-usage rarely does.
Now for your specific question.
The i7-2600 and E3-1245 are priced similarly according to MSRP, though they are aimed at different market-segments. The i7 is the top-tier Desktop part, where the E3 is the value Server part. Both chips are aimed at high-clock, low core environments. That said, the E3 is designed to work with other CPUs in the same system, so the benefits of the CPU stack when you use a dual-socket motherboard.
The top-end of the Server segment is the E7 line. The E7-4870 isn't in the PassMark database, but looking at the spec-sheet you can see how different it is. It has much lower clock than the E3-1245 (2.4GHz vs 3.4GHz), has a much higher core count (10 vs 4), and correspondingly has a much larger onboard cache (30MB vs 8MB). This is a chip designed for high-parallel environments such as virtualization.
Would I go with an i7 instead of an E3?
If I was building a server from scratch and wasn't planning on spending the extra money to get server-grade kit, I would go with the i7. This isn't a server I'd bet my business on, but it would be a quality dev-server.
If I was building a server for revenue service, I'd drop the extra money for server-grade parts and go with two E3's. I want the increased reliability server-grade parts give, and multiple processors give me more head-room to scale should whatever I'm doing start taking off.